Fresh data on the cool way to recycle.

At 25 years old, cryogenic granulation is hardly new, but NPE '94 in Chicago did shine new light on this recycling method, which uses liquid nitrogen to embrittle reclaim before grinding.

During the show's recycling conference sessions, one technical paper detailed some applications where cryogenics works and some where it doesn't.

The show also saw the introduction of a brand-new integrated cryogenic granulation system, a recent joint development of AEC/Nelmor of North Uxbridge, Mass., and Air Products and Chemicals Inc. of Allentown, Pa.

DOLLARS AND SENSE

Refrigerant costs of 1-5[cents]/lb of processed material might well raise doubts about the cost-effectiveness of cryogenic granulation. Yet a paper presented by Nelmor vice president Thomas Tomaszek identified at least one application where the cost would be justified. Tomaszek looked at the effect of subambient processing on three common plastics: post-consumer PET from soft-drink bottles, HDPE reclaim from milk bottles, and flexible PVC from notebook covers. After coarse pregrinding, the materials were run through a Nelmor G810M1 beside-the-press granulator with three different particulate-sizing screens at three different temperature levels--75 F, 0 F, and -320 F.

Not surprisingly, production rates exhibited a direct relationship to screen size. "One can expect a percentile increase and/or decrease in the production of a granulator equal to the percentile difference in the chosen screen-hole diameters," Tomaszek noted.

The effects of temperature were less obvious. Grinding at the subambient temperature levels worked best on PVC, but the value came not from any increase in production rates but from substantial energy savings. Because of the expense of the nitrogen (3.45[cents]/lb in the test), cryogenic granulation fostered an operating-cost advantage over ambient granulation only with a limited range of screen sizes and production rates.

Other advantages turned up during the tests. For one thing, flexible PVC ground at subambient temperatures caused fewer instances of granulator stalling from motor overload. And when processed at lower temperatures, the ground PVC particles showed improved uniformity.

Apart from flexible PVC, Tomaszek believes any amorphous polymer will yield similar energy reductions when processed at subambient temperatures. Other potential applications include separation or delamination of materials with different embrittlement temperatures. As an example, Tomaszek cites the removal of chrome plating from ABS, and he also he considers carpet recycling "a big market opportunity." Finally, any application where thermal degradation is a concern could warrant cryogenics, TABULAR DATA OMITTED Tomaszek adds.

TABLE 1--EFFECT OF TEMPERATURE ON GRANULATION OF THREE COMMON RECLAIM
MATERIALS(a)

                 Screen
                 Diam., in.     Kwh/lb      Lb/hr      Kwh/hr

Flexible PVC

75 F             0.5            0.0429      496.4      21.29556
0 F              0.5            0.0148      544.4      8.05712
-320 F           0.5            0.0111      410.2      4.55322

75 F             0.375          0.1282      448.2      57.45924
0 F              0.375          0.071       392.4      27.8604
320 F            0.375          0.0121      383.2      4.63672

75 F             0.25           0.2374      292.7      69.48698
0 F              0.25           0.1046      214.9      22.47854
-320 F           0.25           0.0431      222        9.5682

HDPE

75 F             0.5            0.0664      260.6      17.30384
-320 F           0.5            0.053       198.3      10.5099

75 F             0.375          0.0806      226.3      18.23978
-320 F           0.375          0.1191      168.3      20.04453

75 F             0.25           0.18        145.9      26.262
-320 F           0.25           0.2547      113.5      28.90845

PET

75 F             0.5            0.1268      242.2      30.71096
-320 F           0.5            0.1337      156.1      20.87057

75 F             0.375          0.1906      204.3      38.93958
-320 F           0.375          0.2039      137.2      27.97508

75 F             0.25           0.482       135.7      65.4074
-320 F           0.25           0.3513      119.1      41.83983

a Flexible PVC from notebook covers; HDPE from PCR milk jugs; PET from PCR
softdrink bottles. (Source: AEC/Nelmor)

Where doesn't cryogenic grinding fit in? "It's not for commodity resins," Tomaszek states. In the cases of HDPE and PET, for example, productivity advantages and energy savings were negligible or too slight to counterbalance the cost of the nitrogen and the higher cost of the cryogenic system itself.

INTEGRATED SYSTEM

The new cryogenic granulator introduced at NPE pairs a liquid-nitrogen supply system built by Air Products with a Nelmor granulator modified to include beefed-up thermal insulation and specialized process controls--including provisions for on-demand feeding. A stainless-steel version handles temperatures down to -100 F, while a carbon-steel model goes down to 0 F.

Working together, Air Products and Nelmor plan to develop a variety of integrated cryogenic granulation systems for hard-to-reclaim materials. Either company can provide single-source responsibility for a turnkey project.